Engines that burn diesel fuel are the most popular type of compression ignition engines. So-called diesel engines introduce fuel at high pressure directly into the combustion chamber. Diesel engines are very efficient because this allows high compression ratios to be employed without the danger of knocking, which is the premature detonation of the fuel mixture inside the combustion chamber. Because diesel engines introduce their fuel directly into the combustion chamber, the fuel injection pressure must be greater than the pressure inside the combustion chamber when the fuel is being introduced, and, for liquid fuels the pressure must be significantly higher so that the fuel is atomized for efficient combustion.
Diesel engines are favored by industry because they are proven performers that are known to give operators the best combination of power, performance, efficiency and reliability. For example, diesel engines are generally much less expensive to operate compared to gasoline fueled spark-ignited engines, especially in high-use applications where a lot of fuel is consumed. However, a disadvantage of diesel engines is that they can produce more pollution, such as particulate matter (soot) and NOx, which are subject to increasingly stringent regulations that require such emissions to be progressively reduced over time. To comply with such regulations, engine manufacturers are developing catalytic converters and other aftertreatment devices to remove pollutants from the exhaust stream. Improvements to the fuel are also being introduced, for example to reduce the amount of sulfur in the fuel, to prevent sulfur from de-activating catalysts and to reduce air pollution. Research is being conducted to improve combustion efficiency to reduce engine emissions, for example by making refinements to engine control strategies. However, most of these approaches add to the capital cost of the engine and/or the operating costs.
Recent developments have been directed to substituting some of the diesel fuel with cleaner burning gaseous fuels such as, for example, natural gas, pure methane, butane, propane, hydrogen, and blends thereof. However, in this disclosure “gaseous fuel” is defined more broadly than these examples, as any combustible fuel that is in the gaseous phase at atmospheric pressure and ambient temperature. Since gaseous fuels typically do not auto-ignite at the same temperature and pressure as diesel fuel, a small amount of liquid fuel can be introduced into the combustion chamber to auto-ignite and trigger the ignition of the gaseous fuel. One approach for consuming gaseous fuel on board a vehicle involves introducing the gaseous fuel into the engine's intake air manifold at relatively low pressures. However, with this approach, engines have been unable to match the performance and efficiency of diesel engines. In a preferred method, it is possible to substantially match the performance and efficiency of a conventional diesel engine by delivering a high-pressure gaseous fuel to an engine for injection directly into the combustion chamber.
A problem with delivering two different fuels for injection directly into the combustion chambers of an internal combustion engine, is that it can be difficult to find the physical space for two fuel injection valves per cylinder and space near the fuel injection valves to provide two high pressure fuel rails in addition to drain lines for taking away fuel that may leak from the fuel injection valves and fluid that is drained from control chambers of hydraulically actuated fuel injection valves.
High-pressure liquid fuel that leaks from a conventional diesel fuel injection valve is normally collected and directed to a drain rail that returns the fuel back to a fuel tank. Such a drain can also be employed to collect diesel fuel that is drained from a control chamber of a hydraulic actuator for the valve needle, when the diesel fuel is also employed as a hydraulic fluid for actuating the fuel injection valve. In a conventional diesel engine, the low-pressure drain rail adds to the piping around the fuel injection valves, but this is manageable with only one fuel. With an engine that is fueled with a liquid fuel and a gaseous fuel, there is a need to drain liquid fuel and vent high-pressure gaseous fuel that leaks from the gaseous fuel injection valve. If gaseous fuel leaks from a gaseous-fuel injection valve and is not collected and somehow vented, the high-pressure gaseous fuel can collect between the fuel injection valve body and the cylinder head, exerting forces on the fuel injection valve that can act against the clamps that are typically employed to hold the fuel injection valve in position. For a common rail direct injection fuel system, the gaseous fuel can be delivered to the fuel injection valve at a pressure of at least 20 MPa (about 3000 psi), and depending upon the engine characteristics, such as its compression ratio, for some engines the desired fuel injection pressure can be even higher. Accordingly, there is a need to provide for a means for venting any gaseous fuel that leaks from the fuel injection valve without adding to the complexity of the piping to and from the fuel injection valves.